The field relates to port constructions for two-stroke cycle engines in which cylinder port openings have edges shaped to reduce clipping of piston rings as the pistons move across the ports during engine operation.
A two-stroke cycle engine is an internal combustion engine that completes a power cycle with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. One example of a two-stroke cycle engine is an opposed-piston engine in which a pair of pistons is disposed in opposition in the bore of a cylinder. The pistons are disposed crown-to-crown in the bore for reciprocating movement in opposing directions. The cylinder has inlet and exhaust ports that are spaced longitudinally so as to be disposed near respective ends of the cylinder. The opposed pistons control the ports, opening the ports as they move to their bottom center (BC) locations, and closing the ports as they move toward their top center (TC) locations. One of the ports provides passage of the products of combustion out of the bore, the other serves to admit charge air into the bore; these are respectively termed the “exhaust” and “intake” ports.
Each port includes one or more arrays of circumferentially-spaced openings through the sidewall of the cylinder. In some descriptions the openings themselves are called ports. However, in this description, a “port” refers to a circular area near an end of a cylinder in which a collection of port openings is formed to permit the passage of gas into or out of the cylinder. The port openings are separated by bridges (sometimes called “bars”) that support transit of the piston rings across the ports.
The pistons are equipped with one or more rings mounted to their crowns. The skirt, lands, and rings of each piston create a seal that prevents gas flow into or out of the port that the piston controls. Any tangential tension of a ring in its constrained state in the bore causes a radial force outward. Thermal deformation due to combustion heat adds to this force. This radial force causes the ring to deflect in an outward radial direction of the bore into the port openings as the ring traverses the port. When the ring must travel in an inward radial direction of the bore back into the bore, which happens as the port closes and also as it opens fully, the ring must be guided radially inward of the bore.
If the geometry of a port edge at the bore surface is not well designed, the distance over which the ring is allowed to move radially inwardly of the bore can be too short, which increases the inward acceleration of the ring, and hence raises the contact force and stress. This adverse motion is called “ring clipping” (or “port clipping” or “port sticking”). Ring clipping causes an overloaded condition in which the lubricant film acting between the bore and an outer ring surface which contacts the bore is pierced and asperities of the ring and bore surfaces begin to contact. This causes undue wear and increases friction, which leads to localized heating and high temperatures. These high temperatures weaken the metals of which the ring and cylinder are constituted. Combined with the high contact stress, this leads to plastic deformation of both the ring and the port opening edges, which disrupts the geometry and roughens the surface texture, exposing more asperities. If the metals are active enough, then fusion can occur. Combined with plastic deformation, this fusion becomes scuffing, evidenced by torn, smeared, folded, and piled ring and/or cylinder material. Maximum contact stress is reduced by limiting the acceleration of the ring into and out of the port openings. Acceleration is reduced by spreading out the radial motion of the ring over time.
FIGS. 1A through 1D illustrate prior art port opening shapes in the bore surface. In each figure, the view is from the interior of a cylinder in a radial direction of the cylinder toward the bore surface. The simplest prior art port opening shape is seen in FIG. 1A, in which a port opening shape 12 includes top and bottom edges 13 and 14 joined by side edges 15. In this regard, the top edge 13 is the edge nearest the TC location of the controlling piston and the bottom edge 14 is the edge nearest the BC location of the controlling piston. The top and bottom edges 13 and 14 are oriented substantially normally to the cylinder axis 16. The side edges 15 are oriented generally longitudinally with respect to the axis. Together, the edges 13, 14, and 15 define a quadrilateral shape. For a given port width W, this provides the highest integral of open area and crankshaft rotation angle (“angle·area product”) which, in turn, yields the maximum open time-area product for any given crankshaft speed. Because the capacity of a port opening to conduct gas flow is directly proportional to its time·area product, this maximizes engine efficiency or power. However, the flat top and bottom edges 13 and 14 cause a ring to move instantly outward into the port opening (and then instantly inward into the bore), resulting in ring clipping at both edges.
Adding corner rounds 18 to the quadrilateral shape as per FIG. 1B yields only a slight improvement over the shape of FIG. 1A. Excursion into the port opening is still substantial as the ring approaches the flat top and bottom edges, producing clipping almost as severe as the straight quadrilateral shape of FIG. 1A.
Other prior art port opening shapes are provided with elliptical or arched top and bottom edges 13, 14 as per FIG. 1C. These may be described by an ellipse or by three circular arcs with a major arc in the middle of the edge connected to two minor arcs in the corners. Either can be fully described with a major and a minor radius. However, an elliptical top or bottom edge with its major radius controlling the ring motion does not spread out the ring motion over a particularly long distance over the entire elliptical shape. Over most of the ellipse the motion of the ring is spread out, but at the last portion, as the elliptical form approaches its major radius, ring acceleration is quite high, thus causing undue wear.
Another port opening shape shown in FIG. 1D goes to the extreme of providing each of the top and bottom edges 13, 14 with a semi-circular shape having a radius equal to half the port width. The semi-circularity does provide smoother ring transitions, which reduces wear but which also reduces the area of the port opening, thereby limiting the angle·area product.
Accordingly, it is desirable to equip an opposed-piston engine for smoother transitions of piston rings across port openings than are presently achievable with prior art port edge constructions. It is desirable to further reduce wear and scuffing caused by ring clipping while at the same time maintaining an angle·area product approaching that of the quadrilateral shape. By reducing the maximum contact stress caused by the surface of the ring pushing against the edge of the port, asperity contact will be reduced, thereby avoiding scuffing and wear and enhancing the durability of the engine. At the same time, the port opening shape should provide an angle·area product approaching that of the quadrilateral shape.